The present invention relates to data storage devices, and more particularly, this invention relates to testing for cross talk instability in magnetic heads and associated hardware.
In a disk drive the MR head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk. The MR head then plays back recorded magnetic signals (bits) which are arranged in circular tracks on the disk.
The MR sensor is a small stripe of conductive ferromagnetic material, such as Permalloy (NiFe), which changes resistance in response to a magnetic field such as magnetic flux incursions (bits) from a magnetic storage disk. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flowing through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage. Conventional MR sensors based on the AMR effect thus provide an essentially analog signal output, where the resistance and hence signal output is directly related to the strength of the magnetic field being sensed.
FIG. 1 illustrates a cross-sectional view of an MR head 100, in accordance with the prior art. As shown, the MR read head 100 includes an MR sensor which is sandwiched between a hard bias layer HB which is in turn sandwiched between first and second shield layers S1 and S2, with insulating gap layers G1 and G2 separating the sensor and the shield layers. The hard bias layer HB typically includes an upper layer 102 and a seed layer 104 therebeneath. One exemplary material commonly employed for the upper layer 102 is CoPtCr with the seed layer being constructed with Cr.
Lead layers L1 and L2 are sandwiched between the hard bias layer HB and shield layer S2 for providing a sense current to the MR sensor. Magnetic fields from a magnetic disk change the resistance of the sensor procircuital to the strength of the fields. The change in resistance changes the potential across the MR sensor which is processed by channel circuitry as a readback signal.
The MR read head 100 is typically mounted to a slider which, in turn, is attached to a suspension and actuator of a magnetic disk drive. The slider and edges of the MR sensor and other layers of the MR read head 100 form an air bearing surface (ABS). When a magnetic disk is rotated by the drive, the slider and one or more heads are supported against the disk by a cushion of air (an xe2x80x9cair bearingxe2x80x9d) between the disk and the ABS. The air bearing is generated by the rotating disk. The MR read head 100 then reads magnetic flux signals from the rotating disk.
FIG. 2 illustrates a simplified cross-sectional view of the MR head 100 showing the hard bias layer HB and the MR sensor thereof. It should be noted that such simplified illustration is not drawn to scale, and includes crude blocks to simplistically show the overlap between the MR sensor and the hard bias layer HB, and the associated fields.
As shown FIG. 2, the hard bias layer HB includes positive poles 204 and negative poles 206. In use, the positive poles 204 and negative poles 206 of the hard bias layer HB produce first electromagnetic fields 208 in a first direction, and further produce a second electromagnetic field 210 in a second direction.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. This GMR effect has also been observed for these types of multilayer structures, but wherein the ferromagnetic layers have a single crystalline structure and thus exhibit uniaxial magnetic anisotropy, as described in U.S. Pat. No. 5,134,533 and by K. Inomata, et al., J. Appl. Phys. 74 (6), Sep. 15, 1993. The physical origin of the GMR effect is that the application of an external magnetic field causes a reorientation of all of the magnetic moments of the ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the multilayered structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. MR sensors based on the GMR effect also provide an essentially analog signal output.
In high density disk drives bits are closely spaced linearly about each circular track. In order for the MR head to playback the closely spaced bits the MR head has to have high resolution. This is accomplished by close spacing between the first and second shield layers, caused by thin first and second gap layers, so that the MR sensor is magnetically shielded from upstream and downstream bits with respect to the bit being read.
An MR head is typically combined with an inductive write head to form a piggyback MR head or a merged MR head. In either head the write head includes first and second pole pieces which have a gap at a head surface and are magnetically connected at a back gap. The difference between a piggyback MR head and a merged MR head is that the merged MR head employs the second shield layer of the read head as the first pole piece of the write head. A conductive coil induces magnetic flux into the pole pieces, the flux flinging across the gap and recording signals on a rotating disk. The write signals written by the write head are large magnetic fields compared to the read signals shielded by the first and second shield layers. Thus, during the write operation a large magnetic field is applied to one or more of the shield layers causing a dramatic rotation of the magnetic moment of the shield layer.
Magnetic recording data storage technologies, particularly magnetic disk drive technologies, have undergone enormous increases in stored data per unit area of media (areal data density). This has occurred primarily by reducing the size of the magnetic bit through a reduction in the size of the read and write heads and a reduction in the head-disk spacing.
However, sometimes the disk drive exhibits irregularities, such as failed reads and/or writes. Often, a manufacturer will have produced hundreds or thousands of drives before the problem is known. The result is that an entire product line may have to be discarded.
For example, it has been found that some AMR and/or GMR heads exhibit instability such that data cannot be properly read from the disk. One cause of instability is thermal stress caused by transient pulses inside the reader induced by the AC write current inside the writer through inductive and capacitive coupling. This phenomenon is called cross talk instability.
During writing, a write current waveform is applied. As the current changes during writing, a magnetic and electric field is generated, not only at the write head, but also at traces within the suspension interconnect coupling the head to the drive preamp. The change in the magnetic field and line voltage gets coupled to the read circuit, causing crosstalk spikes in the read circuit. These spikes create noise in the read signal, leading to errors. Even worse is that the thermal power of these spikes will cause permanent damage of the reader sensor and result in degradation of signal amplitude and bit error rate. This degradation is called crosstalk instability.
The prior art has attempted to detect and measure cross talk instability. U.S. Pat. Nos. 5,121,263 and 5,206,853 perform such methods on an assembled hard drive. These methods require the head to repeatedly read and write test data to the drive medium. The disadvantage of such systems is that the test can only be performed on a fully functional drive.
U.S. Pat. Nos. 5,568,395 attempts to solve the problem by estimating crosstalk using Computer Aided Design (CAD) during the design phase. The problem with this method is that the results are inaccurate, in that it does not test the performance of the actual heads. Further, this method fails to account for variations in materials used to construct the heads, actual (vs. estimated) conductive and shielding properties of the various components, etc.
What is needed is a method of testing of actual heads outside the drive so that problems can be detected and resolved prior to manufacture of complete disk drives. What is further needed is a way to test reader instability caused by writer excitation, and more particularly, instability caused by cross talk instability.
What is also needed is a way to test heads at a component level, thereby overcoming the prior art problem of being unable to qualify a head at a component level after design due to having to test inside the drive. Such prior art approach does not accurately identify whether the head, suspension interconnect, disk media, etc. is the problem.
A process, computer program product, and apparatus for testing a read/write mechanism are provided. The read/write mechanism includes a read circuit and a write circuit. The read circuit includes a read portion of a head and/or associated electrical coupling. The write circuit includes a write portion of a head and/or associated electrical coupling. The read/write mechanism is coupled to an external (hardware or software) module, which sends write signals to the write circuit. The write signals vary the write current in a write circuit of a read/write mechanism for exciting the write circuit. This creates spikes in the read circuit, which can cause instability. The write current can be varied at predetermined intervals based on a pattern. Note that the intervals need not be uniform, and can be selected randomly. The write current can also be varied randomly to resemble a random pattern. Further, the write current can be toggled on and off. This can be done via a gate, and can be based on a pattern or toggled randomly. Additionally, a bias of the read circuit can be changed.
Measurements are taken of a magnetoresistive impedance of a read current of a read circuit of the read/write mechanism. Measurements are also taken of a signal amplitude in the read circuit. The signal amplitude measured may be a track average amplitude (TAA). A bit error rate of the read circuit, and preferably of the overall system, is also measured. The foregoing operations can be repeated for multiple cycles prior to continuing with the process. Further, the foregoing operations may be performed in varying orders of progression.
The read/write mechanism is failed if the signal amplitude changes by a predetermined amount during the varying of the write current. Likewise, the read/write mechanism is failed if the bit error rate changes by a predetermined amount during the varying of the write current. The read/write mechanism may also be failed if the magnetoresistive impedance changes by a predetermined amount during the varying of the write current.
The write-to-read signal coupling of HGA (Head Gimbal Assembly) and HSA (Head Stack Assembly), including GMR sensor, suspension and flex cable on the actuator arm, can be evaluated at component design level. The methodologies successfully simulate GMR cross talk instability inside a drive on magnetic testers. Therefore, a good prediction of drive failure rate can be achieved.
Thus, methodologies described herein overcome the limitations of the prior art by simulating data at the component level for testing one head. Further, these methodologies make it easier to perform failure analysis and find the cause of problems. Because the tests can be performed on the head assembly itself, they are easier to perform. Otherwise the hard drive would have to be programmed to perform the test, which is very difficult. Moreover, some hard drives are limited in the maximum frequencies that can be achieved, so the head may perform differently on other drives. Thirdly, the methodologies allow a user to evaluate a new design and predict the failure possibility in a drive prior to installation in a complete drive, thereby saving considerable time, expense, and human resources.